1. Field of the Invention
This invention relates generally to carbon materials and more specifically to metals primarily comprising carbon.
2. Description of Related Art
Lightweight conductors are very desirable and many ways of making them have been explored. All of the materials considered to date reported have unfortunately manifested flaws that make them impractical.
Graphite is an attractive starting material because it is lightweight, strong within its atomic plane, and chemically stable. However, it's Fermi surface which separates occupied electronic states in k-space from unoccupied ones (frequently, the overlap of the conduction band edge and the valence band edge) is vanishingly small and graphite has only a very small number of unfilled low-energy electronic states. It falls into a category of materials known as semimetals and is a poor conductor (Kittel, Charles, Introduction to Solid State Physics, 6th ed.; John Wiley & Sons, 1986, pgs 212-213). It would be highly desirable to construct a form of carbon that combined the advantageous material properties of graphite with a higher electrical conductivity than graphite has.
Like semiconductors, semimetals can be doped with appropriate impurities to increase or decrease the relative concentration of holes and electrons, thus varying the conductivity. M. S. Dresselhaus and G. Dresselhaus describe the preparation and properties of doping graphite with intercalation compounds in the review article, "Intercalation compounds of graphite", Advances in Physics, 30(2): 139-326, 1981. They describe methods to introduce metals having low ionization energies, for example lithium, potassium, and rubidium, between sheets of graphite. Any outer electrons of the intercalated metal may be pulled into the graphite sheet and thereby increase the carrier concentration and the in-plane conductivity of the doped graphite. A major deficiency in graphite conductors made in this manner is their lack of stability in air. When the doped graphite is exposed to air, the metal atoms leave the graphite structure and react with air to form oxygen containing compounds, and the availability of carriers to the graphite plane is compromised. Intercalated graphite materials appear stable primarily at very low temperatures, far below room temperature. It would be advantageous to develop a form of graphite that had adequate carrier concentration to be a conductor without loss of stability, at room-temperature or in contact with air.
Graphite is composed of carbon atoms arranged in a planar hexagonal array. Each carbon is sp.sup.2 bonded to three other carbons. Perturbation of these bonds can lead to changes in the properties of the material. Harold Kroto reviewed the properties of C.sub.60 and other arrays resulting from introduction of "defects" in the hexagonal array. For example, he describes that ". . . 12 pentagonal "defects" convert a planar hexagonal array of any size into a quasi-icosahedral cage . . . " (H. Kroto, Space, Stars, C.sub.60 and Soot, Science, 242: 1139-1145, Nov. 25, 1988). Materials composed entirely of hexagons to which 12 pentagons are added are known as fullerenes, because some of the structures that occur (particularly C.sub.60 made from 20 hexagons and 12 pentagons) resemble Buckminster Fuller's geodesic domes. Fullerenes are very stable but are not electrical conductors. Solid C.sub.60 in particular is an insulator. Larger fullerenes are also insulating, until at sufficiently large size they take on some semimetal properties, similar to graphite. None of the fullerenes conduct better than graphite and none are planar. Doping fullerenes with alkali atoms to make them electrically conducting has the same problems discussed above for doping graphite. They are not stable in air.
Sumio Iijima constructed nanotubes made exclusively of carbon. Starting with graphite rolled into tubes, he used pentagons to cap the tubes and heptagons to create a negative curvature in the graphitic plane (Sumio Iijima, Growth of carbon nanotubes, Materials Science and Engineering, B19: 172-180, 1993). Iijima used permutations in bond angles, that is introduced pentagons and heptagons into the graphite configuration, to change the physical geometry of nanotubes or carbon spheroids. Since the length of the nanotube is composed of hexagons and the tubes have a large diameter, Iijima's nanotubes are limited to a conductivity comparable to that of graphite.
N. Hamada, et al. (New One-Dimensional Conductors: Graphitic Microtubules, Phys. Rev. Let. 68(10):1579-81, Mar. 9, 1992) predicted that carbon microtubules could exhibit metallic electronic transport properties as a function of the diameter of the tubule and the degree of helicity. Following their techniques, Hamada showed that a tube having an index of 4,4 is metallic. The naming convention of R. Saito, et al. (R. Saito, M. Fujita, G. Dresselhaus, and M. S. Dresselhaus, Appl. Phys. Lett. 60:2204, 1992) is used. A conventional tight binding calculation reveals that this nanotube has 0.07 allowed states for electrons per eV per atom. Conductivity is proportional to the number of allowed states per electron per eV per atom. The tube has a diameter, easily calculated using conventional techniques, of 5.3 .ANG.. Using the same type of calculations, a tube having an index of 8,8 is shown to have a slightly larger diameter of 10.6 .ANG. would have only 0.035 states per electron volt per atom. Utility of Hamada's structures is limited by the low density of allowed electron states and because the nano-tube diameters are so small that they function essentially as one-dimensional structures. One-dimensional structures can exhibit electron localization phenomena which destroys the tube's metallic properties (Anderson, Physical Review, 109:1492, 1958).
It would be extremely advantageous in many ways if a stable, electrically-conducting material that was as lightweight and strong as graphite could be constructed. It would be particularly useful if it could be constructed in a planar configuration and exhibit a density of states at the Fermi energy of at least about 0.1 states per electron volt per atom.